Fine carbon fiber is used in a variety of composite materials, by virtue of its excellent properties such as high strength, high elastic modulus and high electrical conductivity. In recent years, in conjunction with developments in electronic techniques, fine carbon fiber has been considered a promising candidate for electrically conductive filler for producing an electromagnetic wave shielding material or an antistatic material, and has been envisaged as a filler for electrostatic coating to be applied to resin or a filler for a transparent, electrically conductive resin. Also, by virtue of its excellent tribological characteristics and wear resistance, fine carbon fiber has been envisaged to be applied in, for example, electric brushes and variable resistors. In addition, since fine carbon fiber exhibits high electrical conductivity, high thermal conductivity resistance and electromigration resistance, it has also become of interest as a wiring material for forming devices such as a large-scale integration (LSI).
Conventional carbon fiber (e.g., polyacrylonitrile (PAN)-based carbon fiber, pitch-based carbon fiber or cellulose-based carbon fiber), which is produced through carbonization of organic fiber through thermal treatment in an inert atmosphere, has a relatively large diameter of 5 to 10 μm and exhibits low electrical conductivity. Therefore, such carbon fiber is generally employed as a material for reinforcing resin, ceramic material, etc.
In the 1980's, studies were conducted on vapor grown carbon fiber produced through thermal decomposition of a gas such as hydrocarbon in the presence of a transition metal catalyst. Such a process has been successfully employed in production of carbon fiber having a diameter of about 0.1 to about 0.2 μm (about 100 to about 200 nm) and an aspect ratio of about 10 to about 500.
There has been disclosed fine carbon fiber produced through, for example, the following processes: a process in which an organic compound such as benzene, serving as a raw material, and an organo-transition metallic compound such as ferrocene, serving as a catalyst, are brought into a high-temperature reaction furnace together with a carrier gas, to thereby produce carbon fiber on a substrate (Japanese Laid-Open Patent Publication (kokai) No. 60-27700); a process in which carbon fiber is produced in a dispersed state (Japanese Laid-Open Patent Publication (kokai) No. 60-54998 (U.S. Pat. No. 4,572,813); or a process in which carbon fiber is grown on a reaction furnace wall serving as a base (Japanese Laid-Open Patent Publication (kokai) No. 7-150419).
When such fine carbon fiber is thermally treated at 2,000° C. or higher, crystallinity of the carbon fiber is considerably enhanced (i.e., the carbon fiber is readily graphitized), and the carbon fiber exhibits improved electrical conductivity. Therefore, the thus-treated carbon fiber is employed as an electrically conductive filler material; for example, as a filler for resin or as an additive in secondary batteries.
A characteristic feature of a fiber filament of such fine carbon fiber resides in its shape and crystal structure. The fiber filament has a cylindrical structure including a very thin hollow space in its center portion, and a plurality of graphene sheets (hexagonal carbon layers) grown around the hollow space so as to form concentric cylinders in the form of growth rings. Meanwhile, in the case of carbon fiber which has been thermally treated at 2,000° C. or higher, the cross section of a fiber filament of the carbon fiber assumes a polygonal shape, and in some cases, the distance among the graphene sheets enlarge to form void spaces.
Fiber filaments of such fine carbon fiber have a small diameter, and thus have a large aspect ratio. In general, the fiber filaments are entangled with one another, forming fluffy aggregates.
For example, when fine carbon fiber is grown by means of CVD (chemical vapor deposition) on a reaction furnace wall serving as a base, since a raw material containing a metallic catalyst is continuously supplied onto fine carbon fiber grown on the reaction furnace wall, fine carbon fiber is further grown on the previously grown carbon fiber, which serves as a base, to thereby yield a product in a shape like a tree covered with ice.
In the case of the aforementioned fine carbon fiber having a large aspect ratio or branched fine carbon fiber like an ice-covered tree, fiber filaments of the carbon fiber strongly interact with one another, and thus the fiber filaments are entangled or engaged with one another, forming fluffy or blocky aggregates. Therefore, when such fine carbon fiber is mixed with a matrix such as a resin or a ceramic material to thereby prepare a composite material, the fine carbon fiber fails to be uniformly dispersed in the matrix, and the resultant composite material fails to achieve intended electrical, thermal and mechanical characteristics.
When fine carbon fiber of low bulk density containing such fluffy or blocky aggregates is kneaded with a resin to thereby prepare a composite material, and a thin piece of the composite material is observed under an optical microscope or a transmission electron microscope, the thin piece is found to have numerous black dots attributed to the aggregates contained in the fine carbon fiber. The aggregates which are not dispersed in the resin substantially serve as neither an electrically conductive path nor a thermally conductive path in the composite. Therefore, in order to form an electrically conductive path or a thermally conductive path in the composite material, the amount of the fine carbon fiber to be added to the resin must be increased to a level higher than that calculated by means of, for example, simulation. However, when the amount of the fine carbon fiber added to the resin is increased as described above, other problems arise, including reduction of elasticity of the resin, and reduction of adhesion between the resin and a substrate.
Methods to efficiently achieve electrical, thermal and mechanical characteristics of a resin composite material without deteriorating the mechanical characteristics include a method for making fine carbon fiber oriented in a resin. According to this method, even when a small amount of fine carbon fiber is added to a resin, an electrically conductive path or a thermally conductive path can be formed in the resultant resin composite material.
A method for making fine carbon fiber oriented in a resin is molding the carbon fiber and the resin into a product under unidirectional application of pressure. For example, when a mixture of the aforementioned fine carbon fiber and resin is subjected to molding under pressure by use of an extruder, the fine carbon fiber can be oriented in the resin in the extrusion direction (Japanese Laid-Open Patent Publication (kokai) No. 2001-250894). In this method, since shear stress arises during the course of kneading, aggregates of carbon fiber filaments can be disintegrated to a certain extent. However, in this method, the aggregates are not positively disintegrated, and therefore, some aggregates still remain in the molded product. When fiber filaments of the carbon fiber oriented in the molded product are observed in detail under a microscope, the fiber filaments in the resin are found to be oriented in a direction parallel to the extrusion direction as a whole, but most of the thus-oriented fiber filaments are found to be inclined within an angle of ±30° with respect to the extrusion direction. Therefore, in order to improve characteristics of the resin composite material, the orientation degree of the carbon fiber must be enhanced.
Another method for making fine carbon fiber oriented in a resin is adding fine carbon fiber to a resin such as a thermosetting resin, and applying a magnetic field to the resultant mixture (Japanese Laid-Open Patent Publication (kokai) No. 2002-88257, Japanese Laid-Open Patent Publication (kokai) No. 2001-322139, and Japanese Laid-Open Patent Publication (kokai) No. 2002-273741). This method utilizes a characteristic feature of fine carbon fiber; i.e., magnetism in the axial direction of the carbon fiber differs from that in the radial direction thereof. The diamagnetic susceptibility in a direction parallel to the axial direction of the carbon fiber is lower than that in the radial direction thereof, and in general, a diamagnetic substance repels a magnetic field, to thereby stabilize energy in the system. Therefore, the carbon fiber axis is oriented in a direction of a magnetic field; i.e., a direction in which diamagnetic magnetization is reduced to a minimum level.
Conventional fine carbon fiber such as carbon nano-tube has a very high aspect ratio and a curved structure. Therefore, when such conventional fine carbon fiber is mixed with a resin, even if a magnetic field is applied to the resultant mixture, the carbon fiber fails to achieve sufficient orientation. In the case of fine carbon fiber containing aggregates of fiber filaments, fiber filaments which adhere to one another by van der Waals force can be oriented by means of a magnetic field, but fiber filaments which are entangled with one another fail to be oriented. Meanwhile, in the case of branched fine carbon fiber, branched fiber filaments interfere with one another, thereby inhibiting rearrangement of the filaments in the direction of a magnetic field. Therefore, when a molded product is formed from such fine carbon fiber, anisotropy of physical properties of the molded product; i.e., the ratio of properties in the direction of a magnetic field to those in a direction perpendicular to the magnetic field, becomes small.